Add ability to pass stale-while-revalidate option to cache control

closes TryGhost/Team#2094
This value can be used for non-crucial data with a `public` option. For example: `public, max-age=1, stale-while-revalidate=9`.
The idea behind this option is that the browser would cache the value for the number of seconds in `max-age` and would use it for the number of seconds in `stale-while-revalidate` until it gets a "validated response" from the server. The behaviour should be almost unnoticeable for the end user but would make a big difference in the amount of requests to server.

ghost/mw-cache-control/lib/mw-cache-control.js

@@ -12,10 +12,18 @@ const isString = require('lodash/isString');
  * @param {'public'|'private'} profile Use "private" if you do not want caching
  * @param {object} [options]
  * @param {number} [options.maxAge] The max-age in seconds to use when profile is "public"
+ * @param {number} [options.staleWhileRevalidate] The stale-while-revalidate in seconds to use when profile is "public"
  */
 const cacheControl = (profile, options = {maxAge: 0}) => {
+    const isOptionHasProperty = property => Object.prototype.hasOwnProperty.call(options, property);
+    const publicOptions = [
+        'public',
+        `max-age=${options.maxAge}`,
+        isOptionHasProperty('staleWhileRevalidate') ? `stale-while-revalidate=${options.staleWhileRevalidate}` : ''
+    ];
+
     const profiles = {
-        public: `public, max-age=${options.maxAge}`,
+        public: publicOptions.filter(option => option).join(', '),
         private: 'no-cache, private, no-store, must-revalidate, max-stale=0, post-check=0, pre-check=0'
     };
 

ghost/mw-cache-control/test/cache-control.test.js

@@ -34,6 +34,15 @@ describe('Cache-Control middleware', function () {
         });
     });
 
+    it('correctly sets the public profile headers with staleWhileRevalidate', function (done) {
+        cacheControl('public', {maxAge: 1, staleWhileRevalidate: 9})(null, res, function (a) {
+            should.not.exist(a);
+            res.set.calledOnce.should.be.true();
+            res.set.calledWith({'Cache-Control': 'public, max-age=1, stale-while-revalidate=9'}).should.be.true();
+            done();
+        });
+    });
+
     it('correctly sets the private profile headers', function (done) {
         cacheControl('private')(null, res, function (a) {
             should.not.exist(a);